The Syrian and Lebanese governments have denied any involvement in the assassination of former Lebanese Prime Minister Rafik Hariri last February.
There are fears the report could add to Lebanon's political turmoil
Their statements came after a United Nations inquiry into the killing reported that many leads pointed to the direct involvement of Syrian officials.
UN investigators said they had also found evidence of Lebanese collusion in Mr Hariri's death in a bomb attack.
Syria's information minister said the report was "far from the truth".
KEY UN FINDINGS
Assassins had considerable resources and capabilities
Evidence suggests both Syria and Lebanon were involved
Crime was prepared over several months
Hariri's movements and itineraries were monitored
Highly unlikely Syrian or Lebanese intelligence were not aware of assassination plot
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"It is a political statement against Syria that is based on a set of stories by some witnesses who are known for their anti-Syria positions, including media and political figures," Mehdi Dakhlallah told al-Jazeera TV.
"I believe that this report was far from professional. I don't think that it will lead us to the truth. It will be part of this great subterfuge and tension in the region."
The report specifically alleges that pro-Syrian Lebanese President Emile Lahoud's mobile phone received a call minutes before the assassination from Mahmoud Abdul-Al, the brother of a key figure in the inquiry who is said to have close links with the Syrian authorities.
The Lebanese presidency issued a statement rejecting the claim.
"The press office in the presidential palace categorically denies this information, which has no basis in truth and is a part of a pressure campaign against the president," it said.
Syria was the main power in Lebanon until its military withdrawal earlier this year. The BBC's Ian Pannell in Damascus says it is widely believed that the Syrian authorities were angered by Mr Hariri's growing opposition to their influence in Lebanon.
The report into the killing was prepared at the request of UN Secretary General Kofi Annan.
Mr Hariri's assassination - for which the likely motive was political - was complex and had obviously been planned for months, the report says.
As such, it "could not have been taken without the approval of top-ranked Syrian security officials and could not have been further organised without the collusion of their counterparts in the Lebanese security services".
The report says: "Many leads point directly towards Syrian security officials as being involved with the assassination."
The investigators, led by German magistrate Detlev Mehlis, say the Syrian authorities co-operated only to a degree and accuse several interviewees of "trying to mislead the investigation".
It alleges that a letter from Syrian Foreign Minister Farouq al-Shara to the commission contained "false information".
The investigation is not complete, the report says, with several lines of inquiry still to be pursued.
The UN Security Council will be briefed on 25 October by Mr Mehlis, who handed over the report to Mr Annan on Thursday.
The US ambassador to the UN, John Bolton, found the investigation's conclusions "clearly troubling".
Mr Bolton said several countries had been consulting on what steps to take based on the report's findings.
"We're going to study it very carefully, and based on what's in there, decide what our next course of action might be," he said.
Although Syria's President Bashar al-Assad constantly denies any involvement in the killing, the report will add to the sense of impending crisis felt in the country, our correspondent says.
Syria finds itself almost completely isolated, with little support from other Arab nations, and faces the prospect of crippling UN sanctions, he adds.
Troops have been deployed in the Lebanese capital, Beirut, and other cities in case of violence in the wake of the inquiry's findings.
Mr Hariri and 22 others died in the car bomb attack on 14 February. Since then, a series of bomb attacks has targeted anti-Syrian journalists and politicians as well as Christian areas in Lebanon.